The rate constant of intramolecular electron transfer through oligopeptides based on the alpha-aminoisobutyric acid residue was determined as a function of the peptide length and found to depend weakly on the donor-acceptor separation. By measuring the electron-transfer activation energy and estimating the energy gap between donor and bridge, we were able to discard the electron hopping mechanism.
"lowering of the barrier for electron transmission   . As a consequence , the peptides exhibiting the secondary structure with hydrogen bonded network should promote electron transfer more efficiently comparing to non-hydrogen bonded systems like polyprolines. "
[Show abstract][Hide abstract] ABSTRACT: We have investigated the efficiency of electron transmission through thiolated oligoproline derivatives of general formula: Cys-(Pro)(n)-CSA, where CSA is a cystamine linker and n=1-4. The conductance measurements were performed using STM-based molecular junction approach. We have noted that the conductance of the oligoprolines decays exponentially with increasing length of the molecules and the decay constant was 4.3 nm(-1). This indicates that electron transfer is dominated by superexchange mechanism. Based on this observation, we have concluded that the height of the barrier is affected by the specific conformation of the peptide backbone. Such conclusion is supported by the fact that the oligoprolines do not form intramolecular hydrogen bonds, which could provide alternative electron transfer pathways.
[Show abstract][Hide abstract] ABSTRACT: This feature article addresses several novel aspects regarding the peptide-mediated charge migrations, including: i) radical
exchanges with tunable radical types (σ-radical versus π-radical) and electron-transfer (ET)-channel-tunable cooperative proton-coupled
ET (PCET) mechanism, including hydrogen-atom transfer (HAT), single ET-channel PCET, double ET channel PCET, and channel-type-tunable
(σ-channel versus π-channel) PCET; ii) hole hopping migration between the active groups in the side-chains and its controllability;
iii) hole hopping through stepping-stones via a solvated “hole” form; and iv) electron hopping through positively charged
groups as stepping-stones via a solvated electron state. In particular, the controllability of the ET channels (pathways and
types) and solvated-“hole”/“electron”-based relay mechanisms are mainly mentioned. Clearly, this is an important addition
to the well-documented mechanisms for charge migration in proteins. In view of the complexity of protein charge migration,
further exploration on details of the stepping-stone-based relay mechanisms, by considering the properties and structures
of the redox active centers, their intercalators, and the real surroundings, is still needed.
Keywordspeptides-assisting charge migration-hole hopping mechanism-electron hopping mechanism-proton-coupled charge transfer-channel-tunable electron transfer
Frontiers of Chemistry in China 09/2010; 5(3):309-324. DOI:10.1007/s11458-010-0217-2
[Show abstract][Hide abstract] ABSTRACT: Hopping holes: Oligopeptides with a photosensitive charge-injection system were synthesized and irradiated by a nanosecond laser. A multistep electron-transport process occurred which uses aromatic side chains as hole carriers (see scheme). The rates of this process indicate through-space electron transfer (a hopping mechanism). (Graph Presented)
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